Why Do Freeways Come to an Annoying Hault?

The worst thing about Los Angeles is the traffic hands down. As you sit there in a traffic jam, you first wonder what the hold up is, and as an hour of 5 mph traffic passes on a 70 mph speed limit freeway, you think of the cities you could’ve driven to in the time you are stuck in traffic. Stephen J. Beard and Rich Exner from The Plain Dealer try to explain the annoying phenomena in the above infographic. Yeah, traffic patterns are a bit more complex, but oh well. It’s still informative.

[via Cool Infographics]


  • I saw that one, and you’re right, it’s a little simplistic. It’s like my high school friend telling me, any time someone puts on their brakes on the highway, they cause a traffic jam five miles back.

    I did a project a few years back, where I had to develop a dashboard-like display of predicted network loads, based on number of servers, server bandwidth, threads available, threads used by the server software, and predicted volume by time of day. To do this task justice, I had to sit down and learn traffic theory.

    One day, after it made sense to me within the framework of my dashboard project, I was driving home. I began to notice how traffic lights, on and off ramps, and other features of the roadway interacted with traffic, sometimes producing a slowdown, sometimes a full stop, occasionally leading to a long backup. Just like features of the network produced the same effects on network traffic.

    Traffic is much more complicated than the infographic above shows, but it gets the top-level influences about right. Unfortunately, I doubt most traffic engineers (and I use the word “engineer” loosely) have even this small knowledge of traffic theory. Nor have I ever observed anyone watching an intersection with stopwatch and clipboard in hand.

  • chris holden January 26, 2009 at 12:13 pm

    The funnel analogy is not entirely apt, nor is the more general comparison of water pipes and roadways. It breaks down because in water pipes, the smaller the pipe, the higher the pressure, the faster the water goes. The reason the funnel backs up is because there’s somewhere else for the water to go, something not true of cars on the freeway.

  • In the example described in the main part of the image, it is a wave phenomena. The traffic will only be stopped/slowed for a unique finite section of road-space. As cars enter this section they will slow and as they leave it they will speed up. Obviously it gradually dissipates.

    The illustration with the funnel seems to describe a totally different mechanism at work though, just simply down to too much volume for the road to handle, i.e. over saturation. Whereas the wave phenomenon can occur when roads are below 90% saturation.

    Jon, Traffic Planners/Engineers do exist. I personally counted traffic around Victoria Station, London whilst on a summer placement with a transportation consultancy last summer. There are many specialist software packages which model traffic flow, based on traffic surveys and road geometry (width, radii, lanes, type of traffic, speed limits, etc etc)
    The main reason for many roads (especially in the UK) being congested are because of political and economic factors.

  • I love how quickly the basis of this data visualization was shot down… Maybe someone could take the graphic and apply a more logical explanation. Would hate to see such a pretty picture goto waste =)

  • Robert –

    I did not mean to disrespect the diligent traffic engineers among the profession. I was pointing out that I’ve never seen them in action, and I’ve seen many roadways and intersections that could have used some attention. Probably related to those political and economic factors.

    I will say, though, that two badly congested intersections near my home have been improved greatly through a simple change: the traffic lights for the two side streets entering the major artery are no longer both green at once, but instead alternate. This allows both side streets to empty, and prevents people remaining stopped in the intersection.

    I recently saw an interesting video of a bunch of drivers on a closed circular track, and the video showed this wave-like pattern. The cars went around counterclockwise (guess you’d say anticlockwise) and the place along the circumference that was tied up went around clockwise. The video showed the effects of a couple of variables: speed and number of cars. Very interesting.

  • New Scientist posted a great video illustration of shockwave traffic jams:

  • Jon,

    Yeah, sorry my comment didn’t come across too friendly, I realised after I posted it that it probably sounded a little heavy handed.

    What you said about traffic light timings on that intersection is exactly the kind of thing that can be implemented to ease congestion for a relatively low cost. In the UK we have many systems for monitoring traffic flows (from various types of induction loops buried in the road to above ground monitoring systems using microwaves). These enable the traffic lights to vary the green times to the different arms in real time depending on the traffic flow in order to best reduce queues.
    I think because our country has such issues with congestion we have become fairly advanced in traffic light technology!

    In fact, the (UK based) company I was working with, worked on a project to implement a bus priority scheme in New York to give buses priority over cars at junctions.

    Also, that thing about the circular circuit with the wave moving backwards sounds quite interesting. It is something that i don’t think has been properly understood until fairly recently.

  • Pat –

    That may be the video I referred to, but I recall it being much longer, covering several sets of conditions. It’s really cool to see this visually in this way.

  • Hi all – we took a look at the maths of this a while ago:

    Its a pretty interesting topic

  • Marc –

    Neat stuff.

    Robert –

    I detected nothing adverse in your tone, but then, I’m a big oaf myself. Thick skin is a great asset on the web and elsewhere.

  • Mike Cermak February 3, 2009 at 1:53 pm

    one of the best traffic simulations I’ve ever found was also one of the first I ever ran across, here: http://www.traffic-simulation.de/

    play with some of the variables, take the time warp factor down to about 1.5x, and then watch the “fun” onfold